As people continue to go after the biggest fish in the sea, global fisheries are shrinking—both in number and in the actual body size of their catches. But that rapid evolution can be reversed, according to a new 10-year study published today in the journal Proceedings of the Royal Society B.

Previous research has shown that the size of plants and animals harvested from the wild—from cod to ginseng—is actually decreasing two and a half times the rate Mother Nature would dictate. Many scientists pin this on the human tendency to go after the biggest and best food—and our technological ability to do so with extreme efficiency. Although the new study shows the changes are reversible, it also found that the return to normal size was much more gradual, probably taking more than twice as long as the original downsizing.

"There's a good news story in that the evolutionary changes are not permanent—on a contemporary timescale," says David Conover, lead study author and a professor of marine sciences at Stony Brook University (S.B.) in Long Island, N.Y. "But the bad news is that it's slow."

The study tested 10 generations (one of which emerges every year) of Atlantic silverside (Menidia menidia), which were captured in costal waters off Great South Bay, NY and kept comfortable in lab tanks. In groups of the fish that were selectively culled for the largest specimens (which mimics the practice of most fisheries), average body size decreased during the first five generations. Beginning with the sixth, fish were harvested at random, and by the 10th (and final) generation, the shrunken fish groups had almost returned to average size. The researchers estimate that it would take about 12 generations for the body size of this fish to entirely return to its previous dimensions. But for species such as cod, which has five-year generations, recovery time could be 60 years or longer.

The research team also tested an alternative "fishing" model on other groups and only took the smallest fish. In these groups, the body length actually increased, and didn't decrease again once small-fish-harvesting was replaced with random gathering. Might this be a panacea for failing fisheries? Conover suspects that in the wild these fish probably would, in fact, return to their specifically evolved size as they encountered all of the natural predators and food shortages that weren't present in the lab tanks.

Conover and his team were surprised by the findings: "We really didn't think the recovery would be fast enough to be measurable in five generations."

Chris Darimont, an evolutionary ecologist at the University of California Santa Cruz, calls the new study "greatly important" not only for its findings but also because it paves the way for similar experiments in the wild. He notes that in nature, recovery to full size could also depend on other factors, including remaining genetic diversity and natural predators. Does he expect fish and other species to follow a similar 12-generation recovery rate? "I think it might be highly variable, among even fish species," he writes in an e-mail to ScientificAmerican.com.

This study, which was funded by the Institute for Ocean Conservation Sciences (a research organization affiliated with S.B.) may also have finally solved the long-standing debate over whether these size changes were genetic or due to plasticity (individual adaptation to the environment). Conover believes they "definitely" stemmed from genetic changes, but Darimont says there are too many variables out at sea to resolve the issue just yet.

The findings could also be good news for the natural predators of these fish, which, according to a report released this week by Oceana (an international ocean advocacy group), are feeling the pinch of diminished food supplies. Larger fish such as Pacific salmon, bluefin tuna and even Mediterranean dolphins, are stressed and underweight due to the overfishing of their preferred prey, the report says.

Conover was heartened by the findings, but cautions that, "It's still better to avoid these evolutionary changes in the first place." He and Darimont agree that commercial rules should be changed to require fisheries's catches to be limited to small and mid-size fishes.

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